1. Field of the Disclosure
The present disclosure relates generally to implantable device assemblies, instrumentation systems, and methods for accessing and treating multiple levels of the lumbar spine via a minimally-invasive trans-sacral approach (as described in U.S. Pat. No. 6,558,390 which is incorporated herein by reference). More specifically, in one aspect of the disclosure, the present disclosure generally relates to the imposition of a sequence of two or more distractions on a set of two or more adjacent motion segments as part of the provision of therapy to the spine. The therapy may include an objective to stabilize a portion of the spine and may further include using fusion as part of that stabilization.
The distraction process involves setting a minimum distance between a pair of bone anchors and then locking the bone anchors together to prevent the bone anchors from moving beyond that minimum distance between the bone anchors. One set of teachings within the disclosure teaches a way to compress the distance between two adjacent vertebrae by pulling the bone anchors in the two vertebrae towards each other to provide control over the final distraction distance between the vertebrae and to allow for the imposition of compression of the material placed between the vertebrae.
2. Background Information and Related Art
The concept of providing therapy to adjacent motion segments including fusion therapy is addressed in co-pending and commonly assigned U.S. patent application Ser. No. 11/202,655 for Methods and Apparatus for Provision of Therapy to Adjacent Motion Segments published Mar. 16, 2006 as U.S. Pub. No. 2006/0058800 A1 and incorporated by reference herein.
The individual motion segments within the spinal columns allow movement within constrained limits and provide protection for the spinal cord. A motion segment includes two adjacent vertebrae and the disc between them. The discs are important to allow the spinal column to be flexible and to bear the large forces that pass through the spinal column as a person walks, bends, lifts, or otherwise moves. Unfortunately, for a number of reasons referenced in the '655 application, for some people one or more discs in the spinal column will not operate as intended. The reasons for disc problems range from a congenital defect, disease, injury, or degeneration attributable to aging. Often when the discs are not operating properly, the gap between adjacent vertebral bodies is reduced and this reduction in distance causes additional problems including pain.
A range of therapies have been developed to alleviate the pain associated with disc problems. One class of solutions is to remove the failed disc and then fuse the two adjacent vertebral bodies together with a permanent but inflexible spacing, also referred to as static stabilization. Fusing one section together ends the ability to flex in that motion segment. However, as each motion segment only contributes a small portion of the overall flexibility of the spine, it can be a reasonable trade-off to give up the flexibility of a motion segment in an effort to alleviate significant back pain.
Fusion is one type of stabilization. Other forms of stabilization may be used to alter the relative positions of components. Generally, one of the first steps in trying to provide stabilization therapy including fusion therapy is to move adjacent vertebral bodies relative to one another (called distraction) to compensate for the reduction of intervertebral space attributed to the problems with the disc. Depending on the type of therapy that is to be delivered, it may be useful to separate the adjacent vertebral bodies by more than a normal amount of separation.
3. Vocabulary
It is useful to set forth some of the standard medical vocabulary before getting into a more detailed discussion of the background of the present invention. In the context of this discussion: anterior refers to in front of the spinal column (ventral); and posterior refers to behind the column (dorsal); cephalad means towards the patient's head (sometimes “superior”); caudal (sometimes “inferior”) refers to the direction or location that is closer to the feet.
As the present application contemplates accessing the various vertebral bodies and intervertebral spaces through a preferred approach that comes in from the sacrum and moves towards the head, proximal and distal are defined in context of this approach. Consequently, proximal is closer to the beginning of the channel and the surgeon's hand outside the channel and thus towards the sacrum of the patient. Distal is further from the beginning of the channel and the surgeon and thus towards the head of the patient.
While the general concept of distraction can be applied for moving one item apart from another in any dimension, in the context of this application and the claims that follow, distraction is considered in the orientation of the axes of the spinal column so that distraction increases the distance between two adjacent vertebral bodies as measured in the direction of the cephalad/caudal axis of the spine.
One of skill in the art will recognize that a separate process known as subsidence may cause movement of the anchors and the components attached to the anchor relative to the vertebral body that holds the anchor. In some instances, the distance between intervertebral bodies may move due to subsidence or analogous process. From another viewpoint, the distraction between adjacent vertebrae goes to zero when the fusion process connects the two vertebrae together so there is no longer an intervertebral disc space. Thus, when this application refers to fixation of the distraction distance, all that can be controlled with certainty is the distance between the relevant anchors.
The disclosure addresses the controlled movement of bone anchors to either move them further apart from one another or move them closer together. One of skill in the art will recognize that unless otherwise specified explicitly, that motion of anchors will be relative motion that is a mere statement that the anchors are getting closer together or further apart. Thus if one anchor is pulled towards another it means that the relative distance between the two anchors is reduced. It does not mean that one anchor needs to be stationary and one anchor needs to do all the moving or that both anchors are moving relative to some external point of reference. The specific allocation of which anchor is moving relative to an external point of reference such as the operating table may be influenced by other factors such as how the patient is positioned and held on the operating table.